Different types of aluminium alloys have been used in the past for forming a variety of products for structural applications in the aerospace industry. Designers and manufacturers in the aerospace industry are constantly trying to improve fuel efficiency, product performance and constantly trying to reduce the manufacturing and service costs. The preferred method for achieving the improvements, together with the cost reduction, is the uni-alloy concept, i.e. one aluminium alloy that is capable of having improved property balance in the relevant product forms.
The alloy members and temper designations used herein are in accordance with the well-known aluminium alloy product standards of the Aluminum Association. All percentages are in weight percents, unless otherwise indicated.
State of the art at this moment is high damage tolerant AA2×24 (i.e. AA2524) or AA6×13 or AA7×75 for fuselage sheet, AA2324 or AA7×75 for lower wing, AA7055 or AA7449 for upper wing and AA7050 or AA7010 or AA7040 for wing spars and ribs or other sections machined from thick plate. The main reason for using different alloys for each different application is the difference in the property balance for optimum performance of the whole structural part.
For fuselage skin, damage tolerant properties under tensile loading are considered to be very important, that is a combination of fatigue crack growth rate (“FCGR”), plane stress fracture toughness and corrosion. Based on these property requirements, high damage tolerant AA2×24-T351 (see e.g. U.S. Pat. No. 5,213,639 or EP-1026270-A1) or Cu containing AA6xxx-T6 (see e.g. U.S. Pat. Nos. 4,589,932, 5,888,320, US-2002/0039664-A1 or EP-1143027-A1) would be the preferred choice of civilian aircraft manufacturers.
For lower wing skin a similar property balance is desired, but some toughness is allowably sacrificed for higher tensile strength. For this reason AA2×24 in the T39 or a T8× temper are considered to be logical choices (see e.g. U.S. Pat. Nos. 5,865,914, 5,593,516 or EP-1114877-A1), although AA7×75 in the same temper is sometimes also applied.
For upper wing, where compressive loading is more important than the tensile loading, the compressive strength, fatigue (SN-fatigue or life-time) and fracture toughness are the most critical properties. Currently, the preferred choice would be AA7150, AA7055, AA7449 or AA7×75 (see e.g. U.S. Pat. Nos. 5,221,377, 5,865,911, 5,560,789 or 5,312,498). These alloys have high compressive yield strength with at the moment acceptable corrosion resistance and fracture toughness, although aircraft designers would welcome improvements on these property combinations.
For thick sections having a thickness of more than 3 inch or parts machined from such thick sections, a uniform and reliable property balance through thickness is important. Currently, AA7050 or AA7010 or AA7040 (see U.S. Pat. No. 6,027,582) or C80A (see US-2002/0150498-A1) are used for these types of applications. Reduced quench sensitivity, that is deterioration of properties through thickness with lower quenching speed or thicker products, is a major wish from the aircraft manufactures. Especially the properties in the ST-direction are a major concern of the designers and manufactures of structural parts.
A better performance of the aircraft, i.e. reduced manufacturing cost and reduced operation cost, can be achieved by improving the property balance of the aluminium alloys used in the structural part and preferably using only one type of alloy to reduce the cost of the alloy and to reduce the cost in the recycling of aluminium scrap and waste.
Accordingly, it is believed that there is a demand for an aluminium alloy capable of achieving the improved proper property balance in every relevant product form.